Steering system connections in a conventional automotive vehicle are typically provided via direct mechanical linkages between a steering device and the front wheels of the vehicle. For example, in a rack and pinion steering system the rotation of a steering wheel rotates a pinion gear which is engaged with mating notches or teeth in a rack portion of the steering system. Tie rods connect the rack portion to the wheels, so that any rotational motion of the steering wheel ultimately rotates the road wheels to a resultant steering angle, which may vary depending on the steering ratio provided by the steering system. Other steering linkage designs may be used instead of a rack and pinion design, such as worm gears used in a re-circulating ball steering system. In either example, however, all control linkages are purely mechanical in nature.
By way of contrast, in a by-wire steering system an electronically controlled steering actuator is positioned on or in proximity to the front drive axle, and one or more transducers or other sensors measure or detect the steering request or input to a steering device, usually configured as a steering wheel. The sensors can measure a steering angle at the steering device, and/or a rotational force or torque applied to the steering device by an operator of the vehicle. The input signals representing these measured or detected values are then transmitted electrically to a steering actuator, which executes a steering maneuver in response to the signals.
By-wire or electronic steering systems may be configured as a full by-wire system, in which the driver of the vehicle is completely decoupled from the road wheels and all steering input signals are electrically transmitted to the steering actuator. Alternately, a by-wire steering system can be configured as a limited by-wire system, in which a conventional mechanical steering linkage is retained, and in which an electronic steering signal is selectively used to augment or modify a steering response in the steering system.
With respect to limited by-wire systems in particular, such systems can selectively vary a steering ratio to a degree that is determined in part by the speed of the vehicle, and in this manner optimize the steering response. However, during any non-linear maneuver, such as while steering on a low friction surfaces, the overall stability of the vehicle may be less than optimal under certain dynamic conditions.